1. Field of the Invention
The present invention relates to a magnetic recording/reproduction apparatus including a long play mode, and more particularly, to a helical scan system magnetic recording/reproduction apparatus that can record and/or reproduce analog or digital information in a long play mode in addition to a standard play mode.
2. Description of the Related Art
In the field of conventional digital magnetic recording/reproduction apparatuses that record and/or reproduce digital information using a helical scan and azimuth recording/reproduction system, research efforts are expended on employing a long play mode (referred to as LP mode hereinafter) in addition to the standard play mode (referred to as SP mode hereinafter) as recording/reproduction modes. The potential of such a long play mode is discussed in, for example, "A Study on Long Time Mode Recording for Home Use Digital VTR", Proceedings of National Conference of Institute of Electronics, Information and Communication Engineers of Japan, 1-382, Spring, 1990. In such a digital magnetic recording/reproduction apparatus, recording and reproduction of high definition television broadcasting having a great amount of information is desired in addition to the recording and reproduction of the current NTSC system and PAL system television broadcasting. However, a specific structure is not yet known for realizing both the SP mode and the LP mode, or for recording/reproducing various broadcasts differing in amount of information by a conventional digital magnetic recording/reproduction apparatus.
A conventional digital magnetic recording/reproduction apparatus generally has two magnetic heads 2a and 2b with opposite azimuth mounted on the circumferential face of a rotary head cylinder 1, at locations spaced apart by 180.degree. from each other, as shown in FIG. 1. The manner of attaching the magnetic heads to a cylinder is basically similar to that in a VTR of a general VHS system. In the drawing, the positive symbol "+" indicates the azimuth of head 2a (referred to as azimuth A hereinafter), and the negative symbol "-" indicates the azimuth of head 2b (referred to as azimuth B hereinafter).
According to the structure shown in FIG. 1, heads 2a and 2b helically-scan a magnetic tape (not shown) alternately for each half rotation of cylinder 1 by rotating cylinder 1 at a constant speed while winding and running the magnetic tape around cylinder 1 at a predetermined angle in a recording/reproduction operation.
Particularly in a recording/reproduction operation of the SP mode, the travel speed in the tape is controlled so that the scanning position of the head on the magnetic tape is shifted by every predetermined one track pitch substantially corresponding to the width of the head.
In response to a rotation detection pulse (referred to as RF pulse) of high frequency in synchronization with the rotation of cylinder 1, generated by a detection means (not shown), heads 2a and 2b are alternately selected as recording or reproducing heads for every half rotation of cylinder 1.
As a result, in a recording operation of a SP mode, a track 4a of azimuth A and a track 4b of azimuth B are alternately formed on a magnetic tape 3 with a predetermined track pitch t for each half rotation of cylinder 1 in response to the RF pulse, whereby a plurality of blocks of record data D1, D2, D3, . . . , each corresponding to half rotation of cylinder 1, i.e. corresponding to 1 track, are continuously recorded on the formed plurality of tracks, as shown in FIG. 2. The arrows a and b in FIG. 2 indicate the tape travel direction and the head scanning direction, respectively. When the data to be recorded has a great amount of information such as video and audio data, the data to be recorded is compressed to several times smaller than the original amount of data by high efficiency coding, and then recorded.
A digital magnetic recording/reproduction apparatus is envisioned including, in addition to the above-described SP mode, a long play mode that realizes a recording and reproducing time period N times that of the SP mode. In order to realize recording and reproduction in a LP mode that is N times the recording and reproducing time period of a SP mode, basically it is necessary to reduce the tape travel speed in the LP mode to 1/N the tape travel speed of the SP mode. This also applies to the case of the so-called EP mode of a VHS system VTR.
Assuming that the rotation speed of a cylinder is maintained at a speed identical to that of the SP mode while the tape travel speed is reduced to 1/N of that in the SP mode in a conventional digital magnetic recording/reproduction apparatus, tracks will be formed in a sequentially overwritten manner, whereby the track pitch in the recording/reproduction operation of a LP mode is narrowed to 1/N the track pitch of a SP mode. In other words, the track density on a tape in a LP mode becomes N times that in the SP mode to reduce the level of a signal reproduced from each track. This leads to reduction of the S/N ratio of recording and reproduction in the LP mode.
A possible consideration is that the track pitch formed on a tape in a LP mode can be maintained identical to that of the SP mode by compressing the amount of data of the information to be recorded/reproduced at a data compression rate N times that of the SP mode to reduce the original amount of information to 1/N, and by reducing the tape travel speed and the rotation speed of the cylinder to 1/N those of the SP mode. For example, using the heads of the structure shown in FIG. 1, if the tape speed and the rotation speed of cylinder 1 are reduced to 1/N those of the SP mode in the recording and reproducing operation of a LP mode, the track pattern on a magnetic tape will be scanned at a predetermined track pitch t for every half rotation of cylinder 1, similar to the track pattern in the SP mode of FIG. 2.
However, the above-described structure requires a change of the rotation speed of the cylinder in response to switching between the SP mode and the LP mode, giving rise to a problem that a complex control system is needed.
Because the relative speed between the tape and the head in a recording and reproducing operation is significantly lower in the LP mode than in the SP mode, the level of a signal reproduced in the LP mode is greatly reduced to also lower the frequency thereof (1/N). It is therefore necessary to broaden the characteristics of reproduction circuitry such as electromagnetic conversion system and waveform equalization to wide band characteristics that covers frequency as low as 1/N in comparison with the circuitry characteristics where only the SP mode is assumed. Because the output level of a signal reproduced in a LP mode is greatly degraded as described above, the S/N ratio of a reproduced output of head is significantly reduced. One approach in solving these problems involves by setting the rotation speed of the cylinder to 1/N the rotation speed of the SP mode only during a recording operation of the LP mode, and setting the rotation speed of the cylinder to two times that of the reproducing operation, i.e., equal to that of the SP mode, in a reproducing operation of the LP mode. However, in such a case, a magnetic head of a width sufficiently wider than the track pitch must be used to properly trace the track formed on the tape in a reproducing operation. Furthermore, a complex control system is required to control the switching of the rotation speed of the cylinder and for controlling tracking.
Thus, various disadvantages are generated when recording and reproduction is to be carried out at a track pitch identical to that of the SP mode in an LP mode in a conventional digital magnetic recording/reproduction apparatus. If the cylinder rotation speed in an LP mode is maintained identical to that of an SP mode while reducing only the tape travel speed to 1/N of that of the SP mode to solve these disadvantages, the track pitch becomes 1/N that of the SP mode to reduce the level of a reproduced signal, as described before. An approach as set forth in the following solves the above-described problems while maintaining the rotation speed of the cylinder at the same speed.
For the sake of simplicity, a case where the recording/reproducing time period of an LP mode is set to two times that of an SP mode, i.e. N=2, will be described below. In an LP mode, it is assumed that the tape travel speed is reduced to 1/2 of that in the SP mode while maintaining the rotation speed of the cylinder equal to that of an SP mode. The above-described problems of increasing the complexity of the cylinder rotation control and change in the reproduced signal characteristics can be eliminated by a structure in which the scanning position of the head on a tape is not shifted by each track pitch t for every half rotation of cylinder 1 as in an SP mode, but shifted by a track pitch t for every one (=N/2) rotation of cylinder 1. However, the head used for recording and reproduction will be limited to either head 2a or 2b, in other words, only a track of either azimuth A or B is formed on the tape. This means that recording and reproduction of the azimuth system cannot be carried out.
In order to enable an azimuth system recording/reproducing operation, an intermittent recording and reproduction operation can be considered wherein recording/reproduction of two tracks is carried out by heads 2a and 2b during a rotation of cylinder 1, recording/reproduction is not carried out during the next rotation, and recording/reproduction of two tracks is carried out by heads 2a and 2b during a second next rotation of cylinder 1. FIG. 3 shows a track pattern on a tape 3 according to such an intermittent operation. Since the rotation speed of cylinder 1 is identical to that of the SP mode in the example shown in FIG. 3, the frequency of the RF pulse is identical to that of the RF pulse in the LP mode of FIG. 2. In FIG. 3, each of record data blocks D1', D2', D3', . . . is the data of each of record data blocks D1, D2, D3, . . . of the SP mode of FIG. 2 having the amount of data compressed to 1/2 and recorded on each track by every 2 blocks. The period of a RF pulse illustrated by the hatched region in FIG. 3 indicates an idle period where the above-described recording/reproduction is not carried out.
Although recording/reproduction in an azimuth system using heads 2a and 2b is possible according to the structure shown in FIG. 3, a track of azimuth B will be overwritten on a track of azimuth A. Therefore, the pitch of the track of azimuth A is narrowed to 1/2, so that the track pitch of an LP mode cannot be maintained identical to that of an SP mode.
The above-described various problems are expected to occur, not only in a magnetic recording/reproduction apparatus carrying out recording and reproduction in a digital system, but also in a magnetic recording/reproduction apparatus of an analog system. For example, the problem of the track pitch formed on a tape being narrowed in an LP mode to reduce the S/N ratio due to the aforementioned reason will also occur in a helical scan system magnetic recording/reproduction apparatus such as a conventional VHS system VTR or 8 mm VTR that carries out analog recording/reproduction. There is also a problem that the mechanical accuracy of the mechanism of the magnetic head and the rotary cylinder must be extremely critical in order to ensure compatibility in a LP mode between different apparatuses.